1. Field of the Invention
The invention relates to an electronic camera having an automatic focus adjusting apparatus.
2. Related Background Art
Almost all of the recent cameras have the automatic focus adjusting function irrespective of the types of the cameras.
FIG. 1 is a block diagram showing a construction of such a kind of electronic camera.
In FIG. 1, reference numeral 1 denotes a lens unit; 2 a lens driving motor; 3 an iris; 4 an iris driving circuit; 5 a solid-state image pickup element such as a CCD or the like for converting an object image into an electric signal; 6 a solid-state image pickup element driving circuit to drive the solid-state image pickup element; 7 an A/D conversion circuit for A/D converting an image pickup signal generated from the image pickup element 5 into a digital signal; 8 a memory to store an output digital signal of the A/D conversion circuit 7; 9 an ES filter to calculate an ES value (which will be explained hereinlater) indicative of a blur amount from the image pickup signal stored in the memory 8; 10 a system control circuit to control the whole system of the electronic still camera; 11 an image pickup signal processing circuit for executing processes such as gamma (.gamma.) conversion, band limitation, and the like to the output of the memory 8; 12 a D/A conversion circuit to D/A convert a digital output signal of the image pickup signal processing circuit 11 into an analog signal; 13 a frequency modulation circuit to frequency modulate an output of the D/A conversion circuit 12; 14 a recording amplifier to amplify an output current of the frequency modulation circuit 13; 15 a magnetic head; 16 a magnetic sheet as a recording medium; 17 a motor to rotate the magnetic sheet 16; 18 a motor servo circuit to stabilize the rotation of the motor; 19 a photometry element to measure the brightness of the object; and 20 a release switch. A series of photographing operations are started by turning on the release switch 20.
FIG. 2 shows a construction of the image pickup element 5 shown in FIG. 1.
In FIG. 2, reference numeral 501 denotes an interline transfer type solid-state image pickup element; 502 a photodiode for converting the light into the charges and accumulating; 503 a vertical CCD to vertically transfer the charges generated in the photodiode; and V.sub.1 to V.sub.4 transfer electrodes of the vertical CCD 503. The transfer electrode V.sub.1 also functions as a transfer gate to transfer the charges of the odd-number rows of the photodiode. Similarly, the transfer electrode V.sub.3 also functions as a transfer gate to transfer the charges of the even-number rows of the photodiode. The vertical CCD 503 is driven by transfer pulses of four phases. Reference numeral 504 denotes a horizontal CCD to horizontally transfer the charges which are transferred from the vertical CCD 503; and H.sub.1 and H.sub.2 indicate transfer electrodes of the horizontal CCD 504. The transfer electrodes H.sub.1 and H.sub.2 are driven by transfer pulses of two phases. Reference numeral 505 denotes an output amplifier for converting the charges into the voltage and generating; 506 a top drain to drain the unnecessary charges by the reverse transfer; and 507 a bottom drain to drain the unnecessary charges by the forward transfer.
FIG. 3 shows an operation sequence of the electronic still camera with the automatic focusing function. When the release switch 20 is turned on at a time T.sub.0, a series of photographing sequence is started. The optimum iris value A.sub.v and the optimum shutter speed T.sub.v are calculated from the output of the photometry element 19. The iris is set into an open state for a period of time between T.sub.1 and T.sub.2. The lens unit 1 is moved by the lens driving motor 2 to the focusing position in a range from the infinite range to the shortest range in an n-stage step manner or continuously for a period of time between T.sub.2 and T.sub.3. A series of operations such as draining of unnecessary charges, exposure, and reading of the signal charges of n times, that is, the AF operation is executed. The blur amount is calculated from the output of the solid-state image pickup element 5 in the reading operations of the signal charges of n times, thereby calculating the position of the smallest blur amount, namely, the optimum focus position. The iris value is set to A.sub.v for a period of time between T.sub.3 and T.sub.4 and the lens unit 1 is set to the focused lens position. The clearing operation to drain the unnecessary charges to the top drain 506 by the reverse transfer is executed from a time T.sub.4. After that, the regular exposure is performed. Subsequently, the signal charges are read out from a time T.sub.5 and the processed signal is recorded onto the magnetic sheet 16.
FIG. 4 is a diagram for explaining one of the methods of detecting the ES value (edge spread value) indicative of the blur amount (hereinafter, referred to as an ES method). Since the ES value and the ES method have been disclosed in U.S. Pat. No. 4,804,831, they will be merely simply explained here. As shown in FIG. 4A, the edge of the video signal sharply rises in the focused state and slowly rises in the unfocused state. FIG. 4B shows an absolute value D of the differential waveform of the video signal. FIGS. 4C and 4D show delay signals DL.sub.1 and DL.sub.2 of the differential waveform D, respectively. FIG. 4E shows an integral waveform I and indicates a contrast of the edge portion of the video signal. FIG. 4F shows the ES value indicative of a sharpness of the edge which is obtained by dividing D by I.
FIG. 5, shows an example of a construction of the ES filter 9.
In FIG. 5, reference numeral 801 denotes a differential circuit; 802 an absolute value circuit; 803 a delay circuit; 804 an integral circuit; 805 a division circuit; 806 a peak-hold circuit. The value having the largest ES value in the image information is determined to be an ES value of the object.
FIG. 6 shows changes in lens position and ES value when the AF operation is executed in order to obtain the in-focus position.
The lens is continuously conveyed from the minimum position to the maximum position. During the lens conveyance, the image information is accumulated into the solid-state image pickup element 5 every vertical scan period (hereinafter, abbreviated to 1 V). The accumulated image signal is read out and the ES value is obtained from the read-out image information. The position of the largest ES value is determined to be the in-focus position. An axis of abscissa indicates a lens traveling amount. An axis of ordinate indicates a focusing signal (in this case, ES value). A curve in which the focusing position that is drawn in this instance is set to the peak point is called a mountain climbing curve. The mountain climbing curve according to the ES method is steep and a high focus detecting precision is obtained. In the above description, the AF operation is executed by using all of the image information which is supplied. However, since a high processing speed is required in the AF operation, a distance measurement frame having a predetermined area is ordinarily used and the AF operation is performed within the distance measurement frame.
However, since the area of the distance measurement frame is fixed to a predetermined value in the conventional AF operation, for instance, in the case where the distance measurement frame has a wide area as shown in FIG. 7A, for example, when both of a tree of a remote range and a man of a near range are simultaneously located in the distance measurement frame, a plurality of peaks exist in the ES value shown in the diagram. The AF operation is, therefore, unstable with respect to a point that the focal point is set to which one of the peak positions. On the contrary, in the case where the distance measurement frame has a narrow area as shown in FIG. 7B, in many cases, a high frequency component to find out the focal point is not included in the distance measurement frame. The ES value is small. In the worst case, there is a problem such that no peaks exists in the ES value and the accurate AF operation is not performed.